If you have bumpy glass, that makes it easier for grout to seep in...just make sure you put plenty of glue to cover the glass completely. Mac glue dries very clear. GE silicone 2 dries pretty clear. If your glass is flat mac would be great, if it's not I'd use the silicone to get good coverage. It should not look unattractive on the backside. All of my windows look great on backside. I've not made GOG to fit snugly into a window but there isn't any reason why it wouldn't look good. Be sure to show us what you make.

I just scored 2 wooden windows today(2ftx2ft) and am excited to get working on them...even tho I haven't finished my "Dog Whisperer" one as of yet, but just getting over my hysterectomy surgery and am just getting started on a small project today...which is a GOG...whoohoo, feels good to create again. And I concur with what Emtnest says about making sure you coat the pcs well, just not too much as it effects the grout around the glass, so not oozing too bad around the glass....does that make sense?!lol!

What I did was use a 2.8 oz tube of GE Silicone II clear for Window/Door/Attic/Basement and squeezed a solid line of the glue all the way around the underside edge of each piece of glass, taking care not to apply too much because you don't want it to interfere with the area where the grout will be. Then when I grouted, I mixed up the grout on the dry side as opposed to wet and runny. This seems to help also to avoid grout creep. My gog of candles looks quite nice from the back.

Here is a link that might be useful: Silva's blog

There is also the weldbond trick of preventing grout seepage-before you grout take watered down weldbond and go over the entire piece with it taking care to get into the grout spaces. when you grout, it removes the weldbond. The dried weldbond helps keep the grout from seeping under.